For a wind turbine designer, one of the most crucial tasks is creating a blade design that allows a turbine connected to these blades or the rotor to effectively convert wind into electricity. The blades must also be designed properly to withstand inertial forces, aerodynamic forces, and structural forces so as to provide a relatively long service life and safe operation. Like all rotating machines, wind turbines are generators of fatigue, and every revolution of its components including the turbine blades produces a load or fatigue cycle, with each of these cycles causing a small, finite amount of damage that eventually may lead to fatigue cracks or other failures. Modeling may be used in some cases to determine service life of a turbine blade during normal operations. However, modeling has its limitations, including variations in the as-built/manufacture blade and a design and the difficulty in accurately modeling operational conditions with varying and sometimes random loading. As a result, wind turbine blades are typically laboratory tested to determine that their fatigue strength or characteristics are adequate for a desired service life.
Presently, wind turbine blades are fatigue tested in the flapwise direction (i.e., out of the rotor plane or in a direction transverse to a plane extending through the blade) and in the edgewise direction (i.e., in the plane of rotation or in a direction parallel to a plane extending through the blade). For large blades (e.g., greater than 40-meter blade lengths) these two fatigue tests (e.g., two single axis tests) are typically run sequentially, and, to simulate a typical service life of a blade, each test may involve placing a blade through 1 million to 10 million or more load or fatigue cycles, which may take 3 to 12 months or more to complete for each tested direction. There is a trend for wind generator systems to become increasingly larger. Unfortunately, however, the larger blades associated with larger wind generator systems are subjected to greater static and dynamic loads and the facilities required to test these larger blades are also very large as newer generation turbine generators being designed with blades 40 meters or more in length. It is very desirable, and often necessary, to test in advance a proposed blade design to ensure that the design is capable of withstanding the expected loads without structural failure and to evaluate the fatigue resistance of the blade design. The test equipment can also be relatively expensive to purchase and operate. Hence, there is a need for blade testing techniques that are less expensive to use and require less time to complete while still providing accurate fatigue testing results.
As further background on laboratory testing, wind turbine blades are tested by applying loads to the blade in various directions. For example, one type of load is applied in a direction perpendicular to the longitudinal or long axis of the blade and is often referred to as a bending load or as a flap load in the wind turbine field. Another type of load is also applied in a direction perpendicular to the longitudinal axis but also perpendicular to the direction of the applied bending or flap load in order to assess the structural properties of the blade in the transverse or rotational direction. Such loads are often referred to as transverse or lead-lag loads. The load applied to the blade in a given direction may be time-invariant or “static.” Alternatively, the load may be made to vary with time in which case the load is often referred to as “cyclic.” Static loads are generally useful in evaluating the stiffness and ultimate strength of the blade whereas cyclic loads are generally useful in evaluating the fatigue resistance of the blade.
Several different types of test systems have been developed and are being used to apply loads to wind turbine blades. One type of test system uses a linear hydraulic actuator to apply the desired loads to the blade. This type of apparatus is advantageous in that it can be used to apply loads in any desired direction by simply mounting the hydraulic actuators at the desired positions on the blade and by orienting the actuators in the appropriate directions, e.g., for sequential flapwise and edgewise testing. More recently, a resonance test system has been designed and used that provides a resonant actuator for applying loads in the flapwise direction and at the resonant or natural frequency of the test system in the flapwise direction. Since the loading apparatus is attached directly or through compliant linkages to the blade, the test system natural frequency is typically lower than the blade natural frequency, thus the fundamental frequencies of the test system (e.g., blade with hardware) are referred to. Concurrently, in some cases, a transverse load is applied (e.g., a load in the edgewise direction) to the edge of the blade at an offset station to load the blade in the edgewise direction at the same time as the blade is loaded in the flapwise direction to better simulate actual operating loads and hasten testing. However, the transverse load has typically been applied with a forced displacement device with a bell crank or similar device that is attached to the ground plane to provide oscillation in the edgewise or transverse direction. The oscillation in the transverse direction has, to date, been provided at the same frequency used for the actuator applying a flapwise load (e.g., both loads are input at the resonant frequency of the test system in the flapwise direction), and the design of the forced displacement device has limited capability due to the large oil flow, if utilizing hydraulic systems, and displacement requirements. As a result, such fatigue testing systems are possible but may be limited by practical constraints for larger blades (e.g., blades over 40 meters) in which flapwise displacement may be quite large such as up to 6 meters or more.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.